This invention relates to an image scanner or the like for reading a manuscript by photoelectric conversion in a facsimile, digital copying machine and so on.
In order to read a manuscript in an image scanner, the manuscript is moved in the sub-scanning direction relative to an image sensor, or the image sensor, or reversely the sensor is moved in the sub-scanning direction relative to the manuscript. In this case, a step motor is often used for the movement of the sensor or the manuscript. This step motor is first accelerated from the still state to a constant rotation speed, and then kept at the constant speed. When the motor is switched from the acceleration to the constant speed, however, the timing for reading is not matched with that for driving the motor. Various methods have been considered to cope with this problem. One example thereof is proposed in Japanese Patent Laid-open Gazette No. 3-201865. In addition, to easily change the reading magnification, a method for the asynchronism between the read pulse and the drive pulse has been proposed in Japanese Patent Laid-open Gazette No. 62-178063.
These conventional methods, however, have the following drawbacks in reading the manuscript at a high speed.
FIG. 2 is a timing chart to which reference is made in explaining the problems. The motor drive pulse generated at each line drives (i.e. increment) of the motor for moving the manuscript or image sensor. In FIG. 2, T is the exposure time for the optimum amount of light to the sensor. The sensor driving pulse used for the sensor to read the manuscript is changed in pulse width as a1 and a2 in each line in order to be always synchronized with the motor drive pulse. Also, the sensor driving pulse width a1, a2 must be smaller than T, or T&gt;a1, a2. If this pulse width is longer than the sensor driving pulse period T, the sensor is saturated, thus deteriorating the picture quality, and the charge generated in each picture element by photoelectric conversion leaks into a adjacent picture elements, making the correct reading impossible. In addition, this charge leaking effect remains in the next several lines.
When a manuscript of A4 size placed to be vertically long is read at a high speed of two seconds or below, the exposure time T approaches the data transfer time D which is taken for image data read one line before to be transferred from the sensor. Therefore, since D is larger than a1, a2, or D&gt;a1, a2, part of data cannot be taken out, or transferred from the sensor in the exposure time a1, a2, and thus partially read data cannot be used as correct data. In addition, the data remaining not taken out is added to the data read in the exposure time a1, a2. The data read in the exposure time a1, a2 is not used from the first, but when it is added with the data obtained one line before, the charge leaks into the adjacent picture elements of the sensor and has an adverse effect in the next several lines. Since the speed at which data is read from the sensor depends on the transfer ability of the sensor itself, the reading time, or the data transfer time D cannot be decreased.
When a large amount of manuscript such as a Patent Gazette or a long manuscript of recorded computer paper is transmitted to the recipient by facsimile, the manuscript cannot be all read at a constant speed because of transmission conditions and memory capacity. Therefore, the reading speed is decreased to a low value or zero or increased. Thus, it is necessary to read data at the decreased or increased speed. However, as illustrated in FIG. 2, the data that can be read during the acceleration is only the first one line of the three lines, and thus the picture quality is remarkably degraded.
Since it is difficult to change the exposure time as a1, a2 during the high-speed reading time as described above, the exposure time must be kept constant. Thus, the motor speed immediately before the constant speed is only one half of the constant speed because of the motor drive pulse. In order to satisfy this acceleration and deceleration conditions, a large-torque motor must be used, resulting in high cost and large size.
Moreover, in an image reader such as a copying machine, the magnification of the image can be changed by changing the reading speed. In this method, however, since the reading speed is changed for the magnification, the control for the drive motor and so on becomes complicated.
As described above, the high-speed reading of a manuscript encounters with various different problems.